Field of the Invention
The present invention relates to an image formation system, an image density correction method, and an image formation apparatus.
Description of the Related Art
There is a tandem-type image formation system (tandem machine) in which two image formation apparatuses such as printers or photocopiers forming images on paper sheets are connected in series, for example. In this kind of tandem machine, a process for forming images on the front and back sides of paper sheets is shared between different image formation apparatuses, for example, thereby to improve productivity as compared to the case of forming images on the front and back sides of paper sheets by one image formation apparatus. Of the two image formation apparatuses constituting the tandem machine, the image formation apparatus disposed on the upstream side in the paper sheet transport direction will also be abbreviated as “upstream machine,” and the image formation apparatus disposed on the downstream side in the paper sheet transport direction will also be abbreviated as “downstream machine.”
The image formation apparatus uses, as a developer, a toner (one-component developer) or a mixture of toner and carrier (two-component developer) to form a toner image on an image carrier (photoconductor drum), and outputs (transfers) the toner image to a paper sheet in contact with the image carrier at a transfer position. In the image formation apparatus using the two-component developer, the adhesion of the toner image formed on the image carrier increases with rises in in-machine temperature due to continuous printing, and the transfer efficiency is likely to be deteriorated to decrease the density of the toner image output to the paper sheet.
To correct the density decrease, there is known a technique by which a change in humidity and temperature is detected, and when the change exceeds a threshold, the density of a toner patch image after the transfer is measured and the measurement result is fed back to the correction of the density of the toner image (for example, JP 2003-140410 A).
In the foregoing tandem machine, plain paper sheets are continuously printed, the in-machine temperature of the upstream machine rises to a value of room temperature plus 8° C., for example, and the in-machine temperature of the downstream machine rises to a value of room temperature plus 18° C., for example, because the heat of the upstream machine is transferred to the downstream machine. That is, the in-machine temperature of the upstream machine and the in-machine temperature of the downstream machine are different in continuous printing.
When the in-machine temperature of the upstream machine and the in-machine temperature of the downstream machine are different in this manner, the adhesion of the toner image formed on the image carrier varies between the upstream machine and the downstream machine. Accordingly, when an image is formed on the front surface of a paper sheet by the upstream machine and an image is formed on the back surface of the paper sheet by the downstream machine, for example, a density difference occurs between the images. The density difference between the images leads to a density difference between two-facing pages of a bound printed material, for example.
The tandem machine further has a density detection sensor on the downstream side of the downstream machine to detect the density of an output image. However, to make density correction using the results of detection by the density detection sensor, it is necessary to print a density detection pattern on a paper sheet separately from a print job, thereby causing a problem of low productivity. In particular, many users of tandem machine suited for high-volume production place importance on productivity. Accordingly, productivity decline is more problematic than variations in the image density among different print jobs as far as the density difference between the images is stable.